Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, such as a copying machine or a printer.
Description of the Related Art
An image forming apparatus is configured to, for example, form toner images having different colors on a plurality of photosensitive members and superimpose those toner images on one another for transfer onto a recording medium, e.g., a sheet, to thereby form a color image. The image forming apparatus may be configured to directly transfer toner images from a plurality of photosensitive members onto a recording medium, or may be configured to perform a primary transfer from photosensitive members onto an intermediate transfer member and then perform a secondary transfer from the intermediate transfer member onto a recording medium.
This type of image forming apparatus is configured such that toner images formed on a respective plurality of photosensitive members are superimposed on one another precisely on a recording medium. However, there may occur so-called color misregistration in which the toner images are not superimposed on one another on the recording medium due to influences of, for example, tolerance of parts of the image forming apparatus and position variations of parts due to a temperature change at the time of image formation. To address this problem, the image forming apparatus performs control for correcting the color misregistration.
As the color misregistration correction control, for example, a pattern image including measurement images for detecting color misregistration of respective colors is formed on an image bearing member, e.g., an intermediate transfer member, to detect formation positions of the measurement images of respective colors. The image forming apparatus calculates amounts of color misregistration based on relative positions of the measurement images of respective colors, and corrects color misregistration by adjusting formation positions of toner images on respective photosensitive members so that those amounts of color misregistration are reduced.
The pattern image for color misregistration detection is detected by an optical sensor. The optical sensor includes a light emitting unit and a light receiving unit. When the pattern image for color misregistration detection, which is formed on the image bearing member, is detected, the optical sensor emits light from the light emitting unit toward the image bearing member and the pattern image on the image bearing member. The light receiving unit receives the reflected light. The light receiving unit outputs an analog signal in accordance with the light amount (reflected light amount) of the received reflected light. The light receiving unit outputs analog signals having different detection values (output values) for the amount of light reflected by the image bearing member and the amount of light reflected by the pattern image. The image forming apparatus converts the analog signals output from the light receiving unit into digital signals based on a predetermined threshold value. The image forming apparatus detects the relative positions of the measurement images for detecting color misregistration of respective colors on the image bearing member based on, for example, a pulse center of gravity of the converted digital signals and timings of rising and falling of pulses.
When the intermediate transfer member has a low reflectance, the optical sensor has difficulty in detecting an achromatic toner image, e.g., a black toner image having a low reflectance. In view of this, there is proposed a technology of forming a chromatic toner image having a high reflectance on the intermediate transfer member as a base and then forming thereon a measurement image for color misregistration detection with a black toner image to facilitate detection of a black measurement image (U.S. Pat. No. 8,744,325).
Further, the surface state of the intermediate transfer member changes due to a manufacture variation or a variation over time. The change of the surface state of the intermediate transfer member results in a change in reflection state. The change of the surface state of the intermediate transfer member causes the threshold value for converting an analog signal, which is dependent on the amount of reflected light, into a digital signal to be an inappropriate value. Because of this, it becomes difficult to detect the position of the pattern image for color misregistration detection accurately. For example, in a case where the reflection state of the intermediate transfer member changes and the amount of reflected light increases to cause an analog signal to exceed the threshold value, the position of the pattern image for color misregistration detection cannot be detected accurately. To counter this problem, there is proposed a technology of setting the threshold value for converting an analog signal into a digital signal based on the amount of light reflected by the intermediate transfer member (Japanese Patent Application Laid-open No. 2007-148080). The threshold value is set based on the amount of reflected light, and thus it is possible to set an appropriate threshold value.